Electric arc furnaces are well known for melting scrap metal for recycling purposes. An arc furnace typically comprises a container for receiving a scrap metal charge, electrodes spaced a distance from the container, and an electrical power source coupled to the electrodes. The power source induces an electrical discharge between the electrode and the metal charge which produces sufficient heat energy to melt the metal charge.
One common problem associated with electric arc furnaces is that the furnace can produce voltage and current disturbances in the power supply network which supplies the arc furnace. This phenomenon, often called "flicker", arises from large erratic fluctuations in reactive load current through the arc furnace at frequencies up to 25 Hz. When flicker is severe, it can impact on the proper operation of sensitive process loads having a point of common coupling with the arc furnace. Further, due to the sensitivity of the human eye, flicker levels which may not have an impact upon process loads may produce annoying fluctuations in incandescent and fluorescent lamp luminescent levels. Accordingly, North American IEEE and international IEC standards of power quality have been established for flicker and harmonic emission levels acceptable for process loads and incandescent and fluorescent lamps.
Systems have been proposed to compensate for the erratic reactive current swings of the typical arc furnace. In one such system, proposed by L. Gyugyi and R. H. Otto in "Principles and Applications of Static, Thyristor-Controlled Shunt Compensators", IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 5, September/October 1978, a shunt-type VAR generator is used to reduce flicker by introducing reactive currents into the load current to cancel the reactive components of the load current. Each leg of the VAR generator comprises a fixed capacitor in parallel with a thyristor-controlled fixed inductor, and the conduction angle of the thyristor is varied in response to the magnitude of measured reactive load current. However, as the thyristors are positioned in parallel with the power source and the load, it is difficult to control the magnitude of the load current. Therefore, there remains a need for an electric arc furnace which allows the user to control the operating temperature of the furnace without having a deleterious impact on power quality.